I went down this path.
- CRF150 (air-cooled) - NOPE. No power. Just a pig.
- CRF250R - YES! I took it to Precision Concepts and they internally lowered the shocks 2 inches. They cut a new ring groove and clip it. You keep the CG and balance as if it was stock. As he grows, you just rebuild the shocks and take the clip out and it goes right back to stock height. This also helps teach him throttle control and handling a bigger bike while still touching the ground. For reference, my daughter was 5'3-5'4. She did great with it. I ended up selling it because she quit going. The bike sold fast as all the other women/kids in camp LOVED that bike as it had power and it fit them.
Second daughter wanted a bike...
- TTR125 - Nope.
- CRF150 water-cooled/big wheel. - Yes. Great bike. Lots of fun. Good hard-packed bike, still a little bit of a pig in the sand.
I also tried a KLX160 (air cooled) - Nope. It still lacked power.
Try and find a water cooled bike for sure. There is a noticeable power difference.
DEFINITELY, consider a 250 and talk to PC about lowering the shocks if you guys are doing any sand or if you think he's ready for that power but just needs a shorter bike.